Astronomy

Tidally locked, and yet spinning?

Tidally locked, and yet spinning?


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Reading about Uranus almost 90° tilt, I was wondering if some rocky planet with mass concentration at one pole could possibly spin around its own axis, while still being locked to its star? Which means the more massive pole always points toward the star?


I think what you are envisioning is having one pole always pointed towards the primary star. So the planet would rotate about that pole, but then the direction of the pole would change over the course of a year to keep the pole pointing towards a star.

This phenomenon of the direction of the pole changing is called "precession". The Earth does it over a period of 26,000 years, but the pole only traces out a fairly small circle in the sky, rather than twisting about a perpendicular line like you envision.

I think your scenario is improbable at best. Planets are like gyroscopes and resist having their axes of rotation be twisted about a perpendicular axis like that. Even a planet with mass concentrations at both poles (maybe a long cigar shape) to maximize tidal force would probably end up losing the perpendicular rotation to tidal friction, and just end up rotating slowly about an axis nearly parallel to the axis of the orbit.


Tidally locked exoplanets may be more common than previously thought

Tidally locked bodies such as the Earth and moon are in synchronous rotation, each taking as long to rotate around its own axis as it does to revolve around its host star or gravitational partner. New research from UW astronomer Rory Barnes indicates that many exoplanets to be found by coming high-powered telescopes also will probably be tidally locked — with one side permanently facing their host star, as one side of the moon forever faces the Earth. NASA

Many exoplanets to be found by coming high-powered telescopes will probably be tidally locked — with one side permanently facing their host star — according to new research by astronomer Rory Barnes of the University of Washington.

Barnes, a UW assistant professor of astronomy and astrobiology, arrived at the finding by questioning the long-held assumption that only those stars that are much smaller and dimmer than the sun could host orbiting planets that were in synchronous orbit, or tidally locked, as the moon is with the Earth. His paper, “Tidal Locking of Habitable Exoplanets,” has been accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy.

Tidal locking results when there is no side-to-side momentum between a body in space and its gravitational partner and they become fixed in their embrace. Tidally locked bodies such as the Earth and moon are in synchronous rotation, meaning that each takes exactly as long to rotate around its own axis as it does to revolve around its host star or gravitational partner. The moon takes 27 days to rotate once on its axis, and 27 days to orbit the Earth once.

The moon is thought to have been created by a Mars-sized celestial body slamming into the young Earth at an angle that set the world spinning initially with approximately 12-hour days.

“The possibility of tidal locking is an old idea, but nobody had ever gone through it systematically,” said Barnes, who is affiliated with the UW-based Virtual Planetary Laboratory.

In the past, he said, researchers tended to use that 12-hour estimation of Earth’s rotation period to model exoplanet behavior, asking, for example, how long an Earthlike exoplanet with a similar orbital spin might take to become tidally locked.

“What I did was say, maybe there are other possibilities — you could have slower or faster initial rotation periods,” Barnes said. “You could have planets larger than Earth, or planets with eccentric orbits — so by exploring that larger parameter space, you find that in fact the old ideas were very limited, there was just one outcome there.”

“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks,” Barnes said. “And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”

Barnes writes: “These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future.”

Being tidally locked was once thought to lead to such extremes of climate as to eliminate any possibility of life, but astronomers have since reasoned that the presence of an atmosphere with winds blowing across a planet’s surface could mitigate these effects and allow for moderate climates and life.

Barnes said he also considered the planets that will likely be discovered by NASA’s next planet-hunting satellite, the Transiting Exoplanet Survey Satellite or TESS, and found that every potentially habitable planet it will detect will likely be tidally locked.

Even if astronomers discover the long-sought Earth “twin” orbiting a virtual twin of the sun, that world may be tidally locked.

“I think the biggest implication going forward,” Barnes said, “is that as we search for life on any exoplanets we need to know if a planet is tidally locked or not.”

The research was funded by a NASA grant through the Virtual Planetary Laboratory.


Can tidally locked planets become untidily locked?

What would the amount of force to spin it be? Obviously it would be very very hard to do but is it possible? And would spinning it throw its rotation off. Assuming you could get it to spin, would the planet continue to spin or would it eventually overtime slow back down and become relocked?

Tidal locking happens because the mass of a planet is not spread out uniformly. The gravity of the star pulls more strongly on the heavier parts of the planet which eventually slows down or speeds up the planets rotation until the heavier parts are always facing the star. Edit: This is incorrect, see below.

Artificially speeding up the rotation of a tidal locked planet indeed should cause it to become tidally unlocked, but unless you actually change the planets composition and make it completely homogeneous it will eventually just revert back to being tidal locked. This will take a very long time of course, so if your goal is something along the lines of terraforming "just" spinning up the planet every couple million years might be sufficient.


Tidally-Locked Planets More Common than Previously Thought, Astronomer Says

Dr. Rory Barnes, an assistant professor in the Department of Astronomy and Astrobiology Program at the University of Washington, arrived at this finding by questioning the long-held assumption that only those stars that are much smaller and dimmer than our Sun could host tidally-locked planets.

This artist’s conception shows a hypothetical tidally-locked planet with two moons orbiting in the habitable zone of a red dwarf star. Image credit: D. Aguilar / Harvard-Smithsonian Center for Astrophysics.

Tidal locking results when there is no side-to-side momentum between a body in space and its gravitational partner and they become fixed in their embrace.

Tidally-locked bodies such as the Earth and the Moon are in synchronous rotation, meaning that each takes exactly as long to rotate around its own axis as it does to revolve around its host star or gravitational partner.

The Moon takes 27 days to rotate once on its axis, and 27 days to orbit the Earth once.

Earth’s only permanent natural satellite is thought to have been created by an object the size of Mars, known as Theia, slamming into the proto-Earth at an angle that set the world spinning initially with approximately 12-hour days.

“The possibility of tidal locking is an old idea, but nobody had ever gone through it systematically,” Dr. Barnes said.

“In the past, researchers tended to use that 12-hour estimation of Earth’s rotation period to model exoplanet behavior, asking, for example, how long an Earth-like exoplanet with a similar orbital spin might take to become tidally locked.”

“What I did was say, maybe there are other possibilities — you could have slower or faster initial rotation periods.”

“You could have planets larger than Earth, or planets with eccentric orbits — so by exploring that larger parameter space, you find that in fact the old ideas were very limited, there was just one outcome there,”

He said: “planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks.”

“And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked.”

“For example, if Earth formed with no moon and with an initial ‘day’ that was 4 days long, one model predicts Earth would be tidally locked to the Sun by now.”

The results of this work suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future.

Being tidally locked was once thought to lead to such extremes of climate as to eliminate any possibility of life, but astronomers have since reasoned that the presence of an atmosphere with winds blowing across a planet’s surface could mitigate these effects and allow for moderate climates and life.

“I also considered the planets that will likely be discovered by NASA’s next planet-hunting satellite, the Transiting Exoplanet Survey Satellite (TESS), and found that every potentially habitable planet it will detect will likely be tidally locked,” Dr. Barnes said.

Rory Barnes. 2017. Tidal Locking of Habitable Exoplanets. Celestial Mechanics and Dynamical Astronomy, in press arXiv: 1708.02981


Astronomy Test 2

Scientist measure the age of rocks using the properties of natural radioactivity.
Around the beginning the of the 20th century, physicists began to understand that some atomic nuclei are not stable but can spontaneously split apart (decay) into smaller nuclei. The process of radioactive decay involves the emission of particles such as elections, or radiation in form of gamma rays. It's not possible to predict when the decay process will happen.

Half life is the amount of time that it takes for 1/2 of a substance to go away

Bear in mind that crater counts can tell us ONLY the time since the surface experienced a major change.

Seismic waves have shown us more about the layers of the earth!

Internal heat was caused by early bombardment and radioactivity.

Sunlight penetrates to the Earth's lower atmosphere and surface is reradiated as infrared as infrared or heat radiation, which is trapped by the atmosphere. The result is a higher surface temperature for our planet.

Earth would be 60% colder without them

Time on surface: 21h 36 mm 20s

It is over 850 degrees F (hottest in the Solar System).
Due to the greenhouse effect, Venus holds the record for the hottest planet in the Solar System. It goes something like this: The greenhouse effects works on Venus just as it does for the Earth. But since Venus has much more CO2—almost a million times more—the effect is much stronger. Sunlight that diffuses through the atmosphere heats the surface just as on Earth, but the thick CO2 acts as a blanket, making it very difficult for the infrared radiation from the ground to get back into space. As a result, the surface heats up until eventually it is emitting enough to balance the energy it receives from the sun.

Jupiter has a small tilt (3 degrees), which means NO seasons.

The tilt of Uranus is almost 98 degrees. Seasons are meaningless, with very little of Sun's heat, and little internal heat, temperature at -370 degrees F.

Period determined by measuring magnetic field. Jupiter has the shortest day.

In order for a planet to be defined as a planet, it needs to orbit around the sun, and have sufficient mass to form a round shape, and also clear its orbit of debris (which pluto does not meet)

Trojan asteroids is an asteroid with the same orbital period as Jupiter (12 years) that is part of a cluster of either 60 degrees in front of or 60 degrees behind Jupiter.

Asteroids fixed angle in Jupiter's orbit

The most famous comet that comes ever 74-79 years is the Halley Comet.

Remnants of solar system formation.
is a region of the Solar System beyond the planets, extending from the orbit of Neptune


2 Answers 2

Yes. Yes, technically.

You can accurately keep time with the moon, although 'accurate' here doesn't mean much because you're talking about on a scale of either hours (using the moon's passage across the sky) or months (measuring by moon cycles). Both of these are consistent enough so you don't need to worry about fluctuations.

Having a moon revolve around a planet in a day means that it needs to be faster, and that means it needs to be closer. As it happens, the Moon is too large for the Earth to be able to rotate around it in a day with no consequences, but it's totally possible for a smaller moon.


4 Answers 4

I don't know if such an eccentricity is possible given a tidally locked planet. Why ? Because it would mean an irregular planetary rotation. A tidally locked planet is still rotating, the thing is, its rotation has the same length as a year.

If you have too much eccentricity, the planet can't always face its sun from the same side. If this was the case, while away from its sun the planet will spend much more time facing one direction, and then fall down to the perigee at great speed, thus rotating faster to keep the sun in view. That's not going to happen.

The big question is : where is your zone of habitability ? The easiest and most probable is on the "twilight ring". If so, see answers to Planet Tidally-Locked to its star having eclipse day/night cycles?

If your planet is habitable everywhere (still quite possible with a good enought weather system), then by definition a tidally-locked planet will have a hard time getting day/night cycles. Unless.

Remember the problem with eccentricity ?

If you have too much eccentricity, the planet can't always face its sun from the same side. (quoting from the same answer, check)

Then you'd have a "day equals year" planet, not quite tidally locked in the usual sense, but with a wiggle proportionnal to its eccentricity. See the Moon libration as an exemple, but with greater wiggling.

Wikipedia :

Libration in longitude results from the eccentricity of the Moon's orbit around Earth the Moon's rotation sometimes leads and sometimes lags its orbital position.

Some quarter (more or less, given the eccentricity) of the planet will always be day (with the sun still varying in height), the opposite quarter always night, and the others will have that day/night cycle.

Sadly, I have no simulation to offer you right now, but I guess it could be an interesting day/night pattern, adding the fact that for some longitudes the day will be that of a dim sum (at the apogee), while for others the day will se the sun at its closest (at the perigee).


Is there any relation between orbital motion and spin motion?

For the Jupiter, Saturn, and Neptune, the rotation axes are fairly close to aligned with the orbital plane. These giant planets grew by first forming as a terrestrial-sized planet and then sweeping up dust and gas to become a giant. The gas and dust moved at slower than orbital speed. Those giant planets were somewhat akin to a snowball rolling down a hill, growing bigger and bigger as it accumulates snow. The rolling motion -- that's the planet's rotation (as opposed to its orbit).

Uranus is an outlier amongst the giant planets. It's tilted at 97.8° with respect to the orbital plane. One explanation is that something very big smacked into Uranus during the formation of the planets.


For the terrestrial sized planets, their rotational motion is pretty much (1) random and (2) unconnected with orbital motion. Ignoring tidal locking, the rotation of a terrestrial-sized planet is dominated by the last few things that smacked into the planet during the planet's formation. The consensus view on how terrestrial-sized planets form is that the form by fractal aggregation. Little tiny particles of dust collide with one another, in pairs, and sometimes stick to form a slightly bigger (but still tiny) dust particle. These collide and stick, in pairs, with other particles, eventually building up to form little tiny rocks. These collide and stick, in pairs, to form boulders. And so on. Eventually you get a planet. Almost all of the angular momentum comes from the last few collisions. The angular momentum from when the planet was tiny is pretty much washed out, and the orientation is pretty much random.

For Mercury and Venus one cannot ignore tidal locking. Mercury is essentially tidally locked to the Sun. Strictly speaking, tidal locking means that the orbital and rotational periods are the same. Mercury is in a 3:2 resonance, but that's because the 1:1 resonance (true tidal locking) is not as advantageous energy-wise as is the 3:2 resonance. The angular momentum Mercury has been stolen by the Sun. Mercury lost it's original angular momentum rather quickly because Mercury is so close to the Sun.

Venus is, in a sense, also tidally locked to the Sun. Venus (the solid part of Venus) rotates less than one rotation per orbit, and the rotation is retrograde. Venus also has a very thick atmosphere. Parts of this atmosphere rotate much, much faster than does the solid part of Venus, and this atmospheric rotation is retrograde to Venus' body rotation. In other words, Venus' upper atmosphere has a prograde rotation with respect to Venus' orbit. Generalizing tidal locking to mean "the state that the planet (or moon) is one once the primary is done toying around with the planet (or moon)", then Venus too is tidally locked. It's rotation is permanent, or nearly so. (The Sun will engulf Venus when the Sun goes into it's red giant phase, five billion years in the future. So Venus' rotational behavior is not quite permanent.)


So what is "tidal locking"? The gravitational field of the Sun acting on a planet, or of a planet acting on a moon, is not uniform. Gravity is a 1/r 2 force, per Newton's law of gravitation. This means that the gravitational acceleration of a planet toward the Sun / a moon toward a planet varies over the volume of the planet / moon. The primary (Sun in the case of a planet, a planet in the case of a moon) exerts a torque on the secondary (the planet or the moon). This "gravity gradient torque" (google that term) is roughly a 1/r 3 torque. This means that planets close to the Sun are subject to a huge torque compared to those further away. This also means that gravity gradient torque can be a problem for satellites orbiting the Earth, particularly those in low Earth orbit. It's a rather big problem for the International Space Station.

In addition to applying a torque to the secondary non-uniform gravitational field does something else to the planet or moon. It squeezes them. You are familiar with the ocean tides. You may not know this, but the Moon and Sun also result in Earth tides (google that term, and also "solid body tide"). The solid Earth isn't quite as solid as you think. You oscillate up and down by less than a meter every 12 hours because the Moon and Sun squeeze the Earth a tiny bit. The body tides exerted by the Earth on the Moon are much larger than the paltry sub-meter Earth tides that the Moon exerts on us. They were much, much bigger 4.5 billion years ago when the Moon was much closer to the Earth than it is now.

If the secondary is not tidally locked (1:1 resonance), the rotation of the secondary will not match the orbit. The axis along against the object is squeezed varies. The secondary is never perfectly elastic, so some of that changing squeezing will result in heating. The object heats up, and does so asymmetrically. The thermal radiation is asymmetric, so it can carry angular momentum away from the secondary. There's yet another effect. The non-spherical shape into which the secondary is squeezed in turn feeds back into the torque that the primary exerts on the secondary. Put it all together and the secondary eventually settles into a rotation that is in some kind of resonance with the orbital rate. For a nearly circular orbit, the most favored resonance is 1:1, true tidal lock.

For the Jupiter, Saturn, and Neptune, the rotation axes are fairly close to aligned with the orbital plane. These giant planets grew by first forming as a terrestrial-sized planet and then sweeping up dust and gas to become a giant. The gas and dust moved at slower than orbital speed. Those giant planets were somewhat akin to a snowball rolling down a hill, growing bigger and bigger as it accumulates snow. The rolling motion -- that's the planet's rotation (as opposed to its orbit).

Uranus is an outlier amongst the giant planets. It's tilted at 97.8° with respect to the orbital plane. One explanation is that something very big smacked into Uranus during the formation of the planets.


For the terrestrial sized planets, their rotational motion is pretty much (1) random and (2) unconnected with orbital motion. Ignoring tidal locking, the rotation of a terrestrial-sized planet is dominated by the last few things that smacked into the planet during the planet's formation. The consensus view on how terrestrial-sized planets form is that the form by fractal aggregation. Little tiny particles of dust collide with one another, in pairs, and sometimes stick to form a slightly bigger (but still tiny) dust particle. These collide and stick, in pairs, with other particles, eventually building up to form little tiny rocks. These collide and stick, in pairs, to form boulders. And so on. Eventually you get a planet. Almost all of the angular momentum comes from the last few collisions. The angular momentum from when the planet was tiny is pretty much washed out, and the orientation is pretty much random.

For Mercury and Venus one cannot ignore tidal locking. Mercury is essentially tidally locked to the Sun. Strictly speaking, tidal locking means that the orbital and rotational periods are the same. Mercury is in a 3:2 resonance, but that's because the 1:1 resonance (true tidal locking) is not as advantageous energy-wise as is the 3:2 resonance. The angular momentum Mercury has been stolen by the Sun. Mercury lost it's original angular momentum rather quickly because Mercury is so close to the Sun.

Venus is, in a sense, also tidally locked to the Sun. Venus (the solid part of Venus) rotates less than one rotation per orbit, and the rotation is retrograde. Venus also has a very thick atmosphere. Parts of this atmosphere rotate much, much faster than does the solid part of Venus, and this atmospheric rotation is retrograde to Venus' body rotation. In other words, Venus' upper atmosphere has a prograde rotation with respect to Venus' orbit. Generalizing tidal locking to mean "the state that the planet (or moon) is one once the primary is done toying around with the planet (or moon)", then Venus too is tidally locked. It's rotation is permanent, or nearly so. (The Sun will engulf Venus when the Sun goes into it's red giant phase, five billion years in the future. So Venus' rotational behavior is not quite permanent.)


So what is "tidal locking"? The gravitational field of the Sun acting on a planet, or of a planet acting on a moon, is not uniform. Gravity is a 1/r 2 force, per Newton's law of gravitation. This means that the gravitational acceleration of a planet toward the Sun / a moon toward a planet varies over the volume of the planet / moon. The primary (Sun in the case of a planet, a planet in the case of a moon) exerts a torque on the secondary (the planet or the moon). This "gravity gradient torque" (google that term) is roughly a 1/r 3 torque. This means that planets close to the Sun are subject to a huge torque compared to those further away. This also means that gravity gradient torque can be a problem for satellites orbiting the Earth, particularly those in low Earth orbit. It's a rather big problem for the International Space Station.

In addition to applying a torque to the secondary non-uniform gravitational field does something else to the planet or moon. It squeezes them. You are familiar with the ocean tides. You may not know this, but the Moon and Sun also result in Earth tides (google that term, and also "solid body tide"). The solid Earth isn't quite as solid as you think. You oscillate up and down by less than a meter every 12 hours because the Moon and Sun squeeze the Earth a tiny bit. The body tides exerted by the Earth on the Moon are much larger than the paltry sub-meter Earth tides that the Moon exerts on us. They were much, much bigger 4.5 billion years ago when the Moon was much closer to the Earth than it is now.

If the secondary is not tidally locked (1:1 resonance), the rotation of the secondary will not match the orbit. The axis along against the object is squeezed varies. The secondary is never perfectly elastic, so some of that changing squeezing will result in heating. The object heats up, and does so asymmetrically. The thermal radiation is asymmetric, so it can carry angular momentum away from the secondary. There's yet another effect. The non-spherical shape into which the secondary is squeezed in turn feeds back into the torque that the primary exerts on the secondary. Put it all together and the secondary eventually settles into a rotation that is in some kind of resonance with the orbital rate. For a nearly circular orbit, the most favored resonance is 1:1, true tidal lock.


Jupiter’s Gravity

I have read a lot about Jupiter and how it’s gravity is so powerful that it impacts the whole solar system and so on. With 318 times the mass of Earth that is not surprising at all.

However, I was surprised to see that the gravity of Jupiter is only 2.5 times that of Earth. Looks like they measure this on both planets where the atmospheric pressure is at 1 bar.

Seems hard to believe that it is so low, but I understand that gravity drops off rapidly as the surface diameter expands.

Is it best just to think of gravity in terms of mass when considering it’s impact? So in this case, Jupiter’s gravity has 318 times the impact of Earth’s when it comes to things like comets and other objects flying through space? And the 2.5 times number is pretty much irrelevant unless you are visiting near the surface?

#2 Sleep Deprived

Remember, your weight on Jupiter (as quoted) is on its 'surface' which is approx. 45,000 miles from its center. Your weight on Earth's surface is approx. 4,000 from its center. If you were to measure your weight on a (non-moving) surface 45,000 miles from Earth's center, it would be much less than at 4,000 miles - I haven't done the math. Also, Jupiter is a gaseous planet, while Earth is solid - there is a big density difference. I guess what I am saying is that comparing weight on Jupiter and on Earth is almost like comparing apples and oranges - there are large differences in many of the basic aspects of the two planets, so comparing a single effect (weight, eg.) on the two may very well end up appearing odd.

In a sense, one's weight is a 'local' measurement. On a more non-localized scale (scale of the solar system) overall mass is the ruler of gravity.

If you've done the math on how-much-would-I-weigh-45,000-miles-from-Earth, you might want to try how-much-would-I-weigh-5-MILLION-miles-from-Earth. Do the same calc for Jupiter. You should see a HUGE difference. I am too lazy to do that math.

#3 TOMDEY

Yep. you're right. Note that both obey Newton's Gm1m2/R 2 , with R being the distance between centers. So, it all makes sense. Anytime you interpret gravity strength text, be sure to note which they are referencing, surface or centers. or something else. They even often further confuse/camouflage things by measuring expressing distances between surfaces. The principle is so reliable, that you can even gauge gravity strength as only the ball ball below your feet, completely ignoring the shell above (derivation is easy, consequent of any R 2 field). Tom

#4 DaveC2042

Here's something in return.

When a little object orbits a big one, the big one 'wobbles' due to the little one's gravitational attraction. They both actually orbit a point, the barycenter, a bit away from the big one's center.

Jupiter is so massive, that the barycenter of the Sun-Jupiter system (and indeed the whole solar system) is outside the surface of the Sun. This despite the Sun being a thousand times as massive.

I find that amazing and not at all intuitive.

#5 Sleep Deprived

Jupiter is so massive, that the barycenter of the Sun-Jupiter system (and indeed the whole solar system) is outside the surface of the Sun. This despite the Sun being a thousand times as massive.

There are a bunch of physical laws in play here. In the end, it is mostly about Conservation of Energy. One form of energy is angular momentum. In a closed system, we can expect Conservation of Angular Momentum - add up all the bits of matter, take into account their mass, their distance from the center of rotation, and their speed, and that should remain constant. So. the angular momentum we see in the Solar System today is close to that of when it was formed. There will be differences because the Solar System isn't a 100% closed system - stars have passed nearby in the past, etc - but the general idea holds. Also, some angular momentum can be converted into heat when orbiting bodies flex and stretch - this is why Io is such a volcanically active body. Anyway, we should have never been surprised that exoplanets exist. In the star formation process, clouds collapse into stars, but because the clouds are so big they will have a certain amount of angular momentum resulting in a relatively fast-spinning star. It is the same idea that makes a spinning skater spin faster when he/she pulls his/her arms in tight. Anyway, there is so much angular momentum in a dust-cloud a light-year across that if you were to scrunch it all into one body, it may very well spin so fast that the body would fly apart. One way around this is that there are small bits of matter orbiting the star, holding a large amount of angular momentum, thus preventing the star from flying apart. These bodies simply have too much orbital energy to have ever gotten close enough to the star to get gobbled up. THAT'S why we have planets and exoplanets.

I found this little tidbit:

Jupiter has most of the rest of the mass of the Solar System and it is five times as far from the Sun as the Earth. As a result it actually has almost 60% of the Solar System's angular momentum.

Amazing that Jupiter is a small fraction of the Sun's mass, yet it holds most of the angular momentum of the Solar System. Seems counter-intuitive that a small piece of star-schmaltz (Jupiter) holds so much energy in the Solar System. We should expect similar circumstances at other stars.

FWIW, this is highly simplified and the complicated stuff is the devil-in-the-details, but the basic message holds pretty well.

Edited by Sleep Deprived, 16 September 2020 - 07:20 PM.


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AAS: tidally locked exoplanet atmospheres

Post by notFritzArgelander » Fri Apr 16, 2021 6:10 pm

Re: AAS: tidally locked exoplanet atmospheres

Post by ThinkerX » Fri Apr 16, 2021 6:54 pm

something I have started idly wondering about lately.

planet is tidally locked - same side facing the sun.

yet, the planet also rotates on it's axis - say a 40-50 hour period.

planet's axial tilt is about 70 degrees.

This should give a region within 10 degrees of the equator something reasonably close to a normal day/night cycle.

Add a global ocean with a large continent in the night side polar region, smaller continents elsewhere. Heated ocean currents carry warm water to the night side area, hit the continent, and rotate back the other direction.

maybe toss in an oversized (captured) moon in a roughly equatorial orbit to justify the axial tilt and planets rotation. Might also help with things like magnetic fields, plate tectonics. That kind of an orbit. to an observer on the planet, said moon would (always?) appear to be 'half'. maybe.


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